Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
Occasionally, maintenance which requires interruption of the operation of the wind turbine may be required. In order to ensure that a wind turbine rotor cannot rotate during such maintenance, a rotor locking system may be provided. Such a rotor locking system may e.g. comprise one or more bores or apertures in a rotor hub, in which a bolt or pin or similar can be introduced from the nacelle. The rotor may thus be locked with respect to the nacelle. Alternative locking systems are also known.
Generally, such rotor locking systems will require that the rotor be positioned in one of one or more predetermined rotor positions in which the rotor lock can function properly. For example, in case of three-bladed wind turbines, a rotor may be stopped in a “Y”-disposition (also sometimes referred to as “bunny ears” disposition) or in a “reversed Y”-disposition. For both dispositions, three angular positions are in principle available,
In order to slow down a rotor and position the rotor in a predetermined position, it is known to use a mechanical brake acting directly or indirectly on a part of the drive train. It may however be complicated to bring the rotor to a halt in exactly the right angular position using such a brake system. Similarly, if a rotor is already halted, but is not in the right position, it may be difficult to set the rotor into a controlled rotation again, and achieve the correct park position quickly.
U.S. 2009/0162202 discloses a method of positioning a wind turbine rotor in a predetermined angular position by using the generator of the wind turbine as a motor and controlling the torque of the generator when functioning as a motor.
It is an object of the present invention to provide an improved method for positioning a wind turbine rotor allowing a very precise positioning.